Identifying Trends from Streamflow Records—A Case Study

Historical streaniflow records for the Ruby River, in southwestern Montana were analyzed for trends and for changes in the precipitation-runoff relationship. Trends were first identified from plots of the annual values and their accumulated departure from the mean. Double mass analysis, the analysis of residuals from regression, and statistical differences in regression equations also provided evidence of trends and changing conditions. Cyclic patterns were identified which were not explained by the annual variation in precipitation. Past grazing policies and practices were reviewed to see if any cause-effect relationships could be determined.